228 



THE SECOND-CHROMOSOME GROUP 



character. Among the streaks the other mutant characters should 

 be distributed in the same ratio as among all the flies, so that tolerably 

 accurate calculations could be made using streak flies only, but such ex- 

 periments are ineflicient. In the "progeny test" experiment of Muller 

 this difficulty was entirely avoided, since the easily determined presence 

 or the absence of streak from a progeny-test culture was all that was 

 required to classify each parent. Streak is not a character that can 

 be successfully handled without quite extensive experience, and even 

 under the best of conditions there is chance of error. 



In favor of streak is its location, which is very important as the 

 link between star and the rest of the chromosomes. A favorable 

 location far more than doubles the usefulness of a character, other 

 things being equal. In viability and other features streak is satis- 

 factory. 



COMMA. 



(Text-figure 79.) 



ORIGIN OF COMMA. 



In one of the F2 cultures from the cross of dachs by pink, there 

 appeared a mutant character called ''comma" (culture II 9, February 

 6, 1913), which consists of a pair of chitinous 

 thickenings on the anterior dorsal part of the 

 thorax (fig. 79). In shape these thickenings 

 are like a pair of commas, lying back to back, 

 with the blunt tails pointing posteriorly, and 

 depressed below the general level of the 

 surface. This character was confined very 

 largely to the females, of which about 20 per 

 cent showed the character; a few of the males 

 also were conomas. 



From the frequency of the commas it was 

 concluded that the character was an auto- 

 somal recessive, which was either very in- 

 viable or, more probably, failed to show in all 

 those flies which were homozygous. In either 

 case the character was markedly sex-limited in the sense that under 

 like conditions far fewer of the males than of the females showed the 



r»V) o T*o p"!" fiT* 



CHROMOSOME CARRYING COMMA. 



In the F2 of the dachs pink cross, the commas seemed to be distrib- 

 uted at random among the pinks and the wild-types, while none were 

 seen among the dachs. This was interpreted as meaning that the locus 

 is in the second chromosome. To test this point, commas were out- 

 crossed to pink of the third chromosome and to vestigial of the second 

 chromosome and an F2 mass-culture was raised in each case (tables 



Text-figure 79. — Diagram 

 of "comma." 



